plugger2 on "[Plugin: WooCommerce - excelling eCommerce] Additional Cost Per Product Variation (to make it easier for combo variations)"

The woothemes product add-ons extension I was looking at was only released about a week ago:

http://www.woothemes.com/products/product-add-ons/

whereas the gravity forms extension was released in Dec 2011:

http://www.woothemes.com/products/gravity-forms-add-ons/

Perhaps you are using of a different woothemes plugin or extension? When were you using it?